On its way to a full release, Apple is developing several variations of iOS, all with a unique focus and a completely different set of users. Here’s what they are, when they’re used, and by whom.
Apple develops multiple variations of iOS for each version, and each has a unique purpose
The consumer version of iOS is obviously available for free to everyone, but there are many stages of development and testing. Most unique variants of iOS are limited to use within Apple or applicable factories and suppliers.
Before Apple releases its first beta version of an operating system to registered developers, the operating system must go through a series of internal tests to ensure it works properly. For this reason, iOS exists in several variations within Apple, all of which can exist simultaneously.
Some types of iOS are used to test the hardware components of upcoming products, while other variants are used by Apple’s software development teams to create and test different features and improvements to the operating system.
To better illustrate Apple’s practices and general approach to iOS development, here’s a brief overview detailing the differences between the many operating system variants used within the company.
LLDiags – What Apple uses for low-level diagnostics
Perhaps the most basic of all, this version of iOS has virtually no user interface. LLDiags is a specialized variant of iOS intended for low-level diagnostics, most commonly in factories.
Devices running LLDiags only feature a simple diagnostic image, known internally as “Diags”. The diagnostic image displays a basic user interface, which can be used to perform tests or restart the device.
iPhone 8 running Diags – YouTube print screen from AppleDemo video
When not in Diags, devices running the LLDiags version of iOS typically boot directly into Recovery Mode and display nothing other than the Recovery Mode logo. The file system often contains the bare minimum, enough for the device to boot, but not much else.
Apple engineers would interact with these devices through a command line interface, via a connected computer.
NonUI – What Apple’s hardware engineers and calibration machines use
The NonUI variant of iOS was created to separate the work done by Apple’s software and hardware development teams.
The Software Engineering (SWE) team worked on what would become the Release version of iOS, with the user interface that the consumer would see. The hardware team received something else.
Apple’s hardware engineering teams (HWTE) did not have access to the normal user interface. Instead, they relied on an improvised version of the iPhone’s operating system with a rudimentary user interface, intended only for testing hardware components such as buttons, ports, cameras, tops -speakers and sensors.
The following first two iPhones perfectly illustrate the differences between the InternalUI and NonUI variants of iOS.
The two “Acorn” prototypes of the original iPhone illustrate the difference between InternalUI and NonUI
While Apple’s software engineers worked on an iPod-like interface with a click wheel, the hardware development team worked on a version of the operating system that featured simple squares and rectangles as buttons.
These early prototypes ran an operating system known as AcornOS and are commonly referred to as “Acorn prototypes” by collectors.
As the iPhone evolved, so did the NonUI variant of its operating system. Later prototypes of the original iPhone featured a version of the operating system with the arguably inappropriate name “SkankPhone.”
Early versions of SkankPhone’s “About” section listed the operating system name as “OS X” rather than “iPhoneOS”. This perfectly illustrates the separation between Apple’s hardware and software teams.
The company’s internal operating systems are full of jokes, crude drawings, and memes. Some versions even contain pop culture references, such as the internal versions of watchOS which contain references to characters from the series “The Simpsons”.
The SkankPhone app would, for example, display different messages at the bottom of the screen. Some of them include: “Skank is the new black” and “Say hello to the Newton MessagePad 3000”.
iPhone running SkankPhone – Michael MJD YouTube Print Screen
The app would later evolve into SwitchBoard, a rudimentary launcher for NonUI apps and environments such as BurnIn. SwitchBoard exists and works independently, without the need for SpringBoard or UIkit, which are used to display the standard iOS UI on other iOS variants.
Although SkankPhone stuck around for a few more years, the app was eventually discontinued with iOS 6.0, and other internal apps took its place. As the iPhone received more and more hardware features, its internal operating system needed more applications to test new components.
Internal NonUI operating systems also often reflect changes that affect the consumer version of iOS, known as “Release” iOS within Apple. In this regard, NonUI versions after iOS 7.0 adopt a flat appearance in some areas.
In terms of specific features, NonUI versions of iOS often feature a development kernel cache, a development iBoot, and support multiple hardware configurations. This type of iOS may also contain references to unreleased or discarded devices such as the iPhone N89 or iPad J98/J99.
Over the years, Apple has created specific variants of iOS NonUI, intended for use in production lines or component calibration machines such as the Horizon machine. All of these OS versions still fall under the NonUI category, which is still used within Apple and in factories.
Internal User Interface – What Apple Software Engineers Use
Arguably the most desirable and interesting variation of iOS is the internal user interface. As the name suggests, this type of iOS features an internal and often pre-production variation of the user interface found on iOS.
This means that InternalUI versions of iOS may contain implementations of never-before-seen features, unannounced products, experimental test environments, or alternative design choices developed by Apple.
An alternative multitasking view can be seen in an internal version of iOS 4, photos of which were published on a Vietnamese website in 2011.
Multitasking view in iOS 4 could have been very different
In addition to the standard user interface, InternalUI versions of iOS often contain specialized settings that affect the appearance or operation of the operating system. This type of iOS also comes with a large number of applications for internal use, which may contain code names or early prototypes of new operating system features.
Notably, internalUI versions of iOS contain built-in bug reporting options, which are handled through the Radar app. To report a bug, an Apple employee would “drop a speed camera.”
The InternalUI variant of iOS generally supports merged iPhones in production and development, as well as support for development boards and FPGAs in some cases. InternalUI builds also contain debug builds of iBoot.
Internal UI versions of iOS also exist in a separate variant called “Desense”, which often has many of the same internal apps and settings, although the content of this type of iOS can vary significantly.
Every so often, an internal version of iOS makes its way to the general public or to communities of collectors and enthusiasts.
In early 2020, an internal build of iOS 14 from a fused iPhone 11 in development was leaked to a few people and eventually leaked to the press. The release contained references to AppClips, wallpaper collections and widgets.
While AppClips and Widgets both debuted later in 2020 with iOS 14, the Wallpaper Collections feature wasn’t introduced with iOS 16 until two years later. This serves as an indicator of how carefully Apple takes quality control.
VendorUI – What is used in factories for testing
Although InternalUI and the end-consumer iOS variant feature the standard iOS UI in full, the same cannot be said for VendorUI.
VendorUI is a specialized type of iOS sent to relevant factories and suppliers for quality control testing. Although the software features SpringBoard and key elements of the iOS user interface, the operating system often does not have all the apps by default.
This version of iOS is usually sent with a checklist of the apps present in the version. Access to it is heavily restricted to prevent leaks of new features.
In March 2024, a credible private account on X claimed that Apple had begun distributing internal builds of the VendorUI version of iOS 18.
CarrierOS – What Carriers Need to Test Features
CarrierOS is a variant of iOS that is often almost identical to the versions of iOS made available to the general public by Apple.
Distributed to operators around the world, this type of iOS contains the standard end user interface as well as specialized applications and utilities that may be useful to operators.
The operating system contains applications such as “E911Tester”, which allows testing of the E-911 emergency call system on iPhones.
iOS Release – What the End Consumer Can Experience
This is the version of iOS used by almost everyone who owns an iPhone. Unless you’re an Apple engineer, you’ve probably only seen the original versions of iOS, which Apple calls “Release” internally.
Developer betas, public betas, and standard .ipsw files all fall under the Release category because they do not contain any internal tools or applications.
Interestingly, internal distributions of Release iOS also exist. These distributions feature the same file system as public versions of iOS, with the addition of iBoot development in some cases.
What does all this mean?
The existence of different variants of iOS is an indication of Apple’s commitment to the quality of its products. New versions of the operating system go through several rounds of testing, even before the first developer betas are sent out in June each year.
Apple also uses its operating systems for outbound quality control, to ensure that new iPhones are all up to standard and that defective units are not shipped.
